Can you trust your constraints?

SQL Server allows you to temporarily disable your CHECK and FOREIGN KEY constraints. I would not recommend this as part of your regular production schedule, but there are certainly cases where this comes in handy.

One example is if you’re copying thousands of rows from another table, and you already know with 100% certainty that the data doesn’t violate any constraint. The overhead of checking could impact performance of your process; in the worst case making the execution time exceed your maintenance window. Disabling the constraints can result in a tremendous reduction of execution time.

Another example would be the case where you’re importing lots of data from a third party; the raw data might violate some constraints, but these errors are all corrected as part of the process before the constraints are re-enabled. I would personally prefer to import the raw data to a staging table, do the cleansing there and only then copy the data to the production table – which of course takes us back to the first example J.

However, there is a large caveat that many people seem to be unaware of – if you don’t take care while re-enabling the constraints, you might end up disabling the query optimizer’s potential to generate the best possible execution plan for your queries!

Let’s first look at the abridged syntax for disabling end re-enabling a constraint:

ALTER TABLE <tablename>

NOCHECK CONSTRAINT <constraintname>;

ALTER TABLE <tablename>

WITH { CHECK | NOCHECK }

CHECK CONSTRAINT <constraintname>;

The first syntax disables a constraint and the second syntax re-enables it. Note the “WITH {CHECK | NOCHECK}” clause. Specifying WITH CHECK signifies to SQL Server that you want it to check the re-enabled constraint for all rows in the table; if one or more fail to satisfy the constraint, the ALTER TABLE command fails. Specifying WITH NOCHECK (the default for existing constraints) means that existing rows are not checked. This is of course faster, but it has a severe side effect that you should really be aware of: you may know with 100% certainty that all rows in the table still abide by the constraint, but SQL Server doesn’t know this. As soon as you enable a constraint without checking the existing rows, SQL Server will mark the constraint as “not trusted”. This means that the query optimizer will no longer use it’s knowledge of the constraint to optimize your queries.

To see the effects of this, I set up a simple test with one table, three columns, and a CHECK constraint:

CREATE TABLE dbo.Test

(KeyColumn int NOT NULL,

CheckColumn int NOT NULL,

LongColumn char(4000) NOT NULL,

CONSTRAINT PK_Test PRIMARY KEY (KeyColumn),

CONSTRAINT CK_Test CHECK (CheckColumn > 0)

);

INSERT INTO dbo.Test (KeyColumn, CheckColumn, LongColumn)

SELECT 1, 1, REPLICATE(‘a’, 4000)

UNION ALL

SELECT 2, 2, REPLICATE(‘b’, 4000)

UNION ALL

SELECT 3, 3, REPLICATE(‘c’, 4000)

UNION ALL

SELECT 4, 4, REPLICATE(‘d’, 4000)

UNION ALL

SELECT 5, 5, REPLICATE(‘e’, 4000)

UNION ALL

SELECT 6, 6, REPLICATE(‘f’, 4000)

UNION ALL

SELECT 7, 7, REPLICATE(‘g’, 4000);

go

If I now execute a query that, because of the CHECK constraint can’t possibly return any rows, I’ll get an execution plan that doesn’t even touch the table:

SELECT KeyColumn, CheckColumn

FROM dbo.Test

WHERE CheckColumn < 0

AND LEFT(LongColumn, 5) = RIGHT(LongColumn, 5);

But let’s now examine what happens if I, for whatever reason, disable and later re-enable the constraint:

ALTER TABLE dbo.Test

NOCHECK CONSTRAINT CK_Test;

— Imagine something actuallly happening here

ALTER TABLE dbo.Test

CHECK CONSTRAINT CK_Test;

Because I didn’t specify the “WITH [CHECK | NOCHECK]” clause when re-enabling the constraint, the command defaults to not checking the existing population. As a result, SQL Server now feels that it can no longer trust this constraint, as can be seen in the catalog view sys.check_constraints:

SELECT LEFT(name, 20) AS name, is_not_trusted

FROM sys.check_constraints;

name is_not_trusted

——————– ————–

CK_Test 1

Since SQL Server no longer trusts this constraint, running the query that searches for negative values in CheckColumn will now cause an execution plan to be created that scans the entire table:

To prevent this from happening, always make sure to use WITH CHECK when you re-enable a constraint:

ALTER TABLE dbo.Test

WITH CHECK CHECK CONSTRAINT CK_Test;

It looks a bit silly at first, the CHECK from the WITH CHECK option directly followed by the CHECK command to re-enable the constraint – but hey, I didn’t create the syntax, you know! You’ll get used to it, eventually.

Anyway, this results in the constraint being trusted again. The example query above will now, once more, return an empty result set without ever actually reading as much as a single page of the table’s data.

SELECT LEFT(name, 20) AS name, is_not_trusted

FROM sys.check_constraints;

name is_not_trusted

——————– ————–

CK_Test 0

Now whereas this example might seem a bit contrived, it should be noted that this also applies to FOREIGN KEY constraints. I chose to use a CHECK constraint here to keep the sample code relatively short, but using a trusted FOREIGN KEY constraint, the optimizer can actually completely remove joined tables from a query! That is of course much more a real-world scenario than searching for a value that’s not permitted – and one that can have an even bigger impact on performance!!

So the bottom line is to always make sure that you include the WITH CHECK option when re-enabling a constraint. Or, to conclude with an awful pun, always double-check that there’s a double CHECK in the command.

>Can you help me out and provide an example please, as I’ve never seen this before on a plan. much appreciated!<<

Sure, no problem. The elimination of a join can (of course) only happen if no columns from the eliminated table are used, i.e. if the join is only needed to verify the existance of a row in the joined table. Here are two (semantically equivalent) queries for use in the AdventureWorks database that return address info for employees that actually exist in the employee table:

USE AdventureWorks;

go

SELECT ea.EmployeeID, ea.AddressID

FROM HumanResources.EmployeeAddress AS ea

INNER JOIN HumanResources.Employee AS e

ON e.EmployeeID = ea.EmployeeID;

SELECT ea.EmployeeID, ea.AddressID

FROM HumanResources.EmployeeAddress AS ea

WHERE EXISTS

(SELECT *

FROM HumanResources.Employee AS e

WHERE e.EmployeeID = ea.EmployeeID);

go

If you check the execution plan, you’ll see that the emplyee table is not used at all; because of the foreign key constraint, the optimizer can safely skip this. Now turn the constraint into a not trusted one:

ALTER TABLE HumanResources.EmployeeAddress

NOCHECK CONSTRAINT FK_EmployeeAddress_Employee_EmployeeID;

ALTER TABLE HumanResources.EmployeeAddress

WITH NOCHECK CHECK CONSTRAINT FK_EmployeeAddress_Employee_EmployeeID;

Rerun the queries above, and both will show an execution plan that includes a join to the Employee table, to check if the employee referenced in the EmployeeAddress table really exists. Restore the trusted state of the constraint to get the original plans again:

No, you can witness the exact same behaviour on SQL Server 2000. All the code above (except the query against sys.check_constraints – see below) can be executed unchanged on SQL Server 2000, and the results will be completely identical.

The only exception, as noted, is that there’s no easy way to check if a constraint is trusted in SQL Server 2000. The only way I was able to find was to check the undocumented bits in the column "sysobjects.status", with this query:

SELECT name, status FROM sysobjects WHERE type = ‘C’;

When I tested this, the value for status was 2 when the constraint was enabled and trusted, 2050 (2 + 2048) when enabled and not trusted, and 2306 (2 + 256 + 2048) when disabled and not trusted.

The is_not_trusted bit will always remain = 1 if the is_not_for_replication bit = 1. The only way to fix this is by dropping the FK Constraint and rebuilding it, but this can be very difficult because when you rebuild it you have to make sure all other options remain the same while only changing those 2. Also, consider there may be some "unknown" valid reason the top level application sitting atop the database wants it this way AND/OR if changing it may violate some support from the application vendor.

The relevant change is the second join. You write that APL_X_mapping_ID is unique, so every row in the ampping table joins to itself. A very strange query.

One possible explanation is if APL_X_mapping_ID is nullable. NULL is not equal to NULL, so SQL Server will have to get the row and check for that – those rows would be rejected from the query result.

Another possible explanation is that you probably don’t have a foreign key on this column. That would not change anything for the data model because a foreign key from a column to itself is useless, but perhaps the logic in the optimizer that removes useless joins only responds to foreign key constraints? (this is speculation on my part)

@Henrik (Sorry for the late reply):

Douglas’ query produces a result set that contains the SQL you have to run. So you first run Douglas’ query (with Results to Text mode), then copy the result set, paste it in a new window and run that.

Do check the SQL produced first. Douglas’ query does not escape table and constraint names, so you are vulnerable to injection or (more probable) will get errors if you have table/constraint names that have spaces.

After running the second query, do check the result for error messages.

@Håkan:

Simple answer: no.

Indexes can be used to quickly retrieve rows based on a value.

(Trusted) constraints can be used to simplify query logic before even starting to retrieve rows, because some conditionas are guaranteed to hit.

Okay, slightly more nuanced answer: if you have untrusted constraints and they cause performance issues, an index can sometimes alleviate the problem. But that’s like telling someone who shoots himself in the foot every Saturday to apply a band aid and take a pain killer. Why not take away his gun or teach him to aim? (In other words, instead of using that index you should fix the root cause, which is the untrusted constraint).

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